(a) Field of the Invention
The present invention relates to process for preparing a high-molecular weight acrylamide polymer which is mainly useful as a high molecular flocculant and as a papermaking chemical. More specifically, it relates to a process for preparing a high-molecular weight acrylamide polymer by initiating polymerization by the use of an oxidation-reduction system polymerization initiator comprising two or more kinds of oxidizing agents.
(b) Description of the Related Art
When an acrylamide polymer is used as a flocculant, it is preferable that the molecular weight of the polymer is as high as possible, so long as handling properties are not impaired, because the polymer having a high molecular weight can exert an excellent flocculation effect and the employment of such a polymer conveniently permits the decrease of its amount to be used. Furthermore, also in the field of a paper strength improver, the polymer having a high molecular weight has been desired in recent years.
Heretofore, a high-molecular weight acrylamide polymer has usually been prepared by carrying out polymerization at the lowest possible polymerization temperature and at the lowest possible concentration of a polymerization initiator. However, when the molecular weight is high, a polymerization solution tends to be in a gel state at the time of the initiation of the polymerization, and immediately stirring becomes impossible and the control of the temperature also becomes impossible. Therefore, in order to control the polymerization temperature to a low level, the polymerization initiation temperature must be low, and in order to inhibit heat generation by the polymerization, the concentration of a polymerization product must also be controlled to a low level.
That is to say, for the preparation of the high-molecular weight acrylamide polymer, the concentration of the polymerization initiator, the polymerization initiation temperature and the concentration of the polymerization product must all be controlled to low levels, and when they are at such low levels, a lengthy period is required for the completion of the polymerization, with the result that the productivity of the polymer deteriorates inconveniently.
When the concentration of the polymerization initiator is merely heightened so as to increase the polymerization rate, the molecular weight of the product decreases. There has not yet been found a technique for preparing a high-molecular weight acrylamide polymer with a high productivity by cutting off the above-mentioned interrelation.
Furthermore, Japanese Patent Application Laid-open No. 268304/1992 discloses a method for preparing a low-molecular weight acrylate polymer such as sodium acrylate and describes the use of a peroxide polymerization initiator and a persulfate in examples, but its effect is not referred to anywhere. Furthermore, in a method for preparing a maleic acid copolymer in Japanese Patent Application Laid-open No. 124711/1991 and a method for preparing a fumaric acid copolymer in Japanese Patent Application Laid-open No. 124712/1991, it has been implied that a peroxide polymerization initiator and a persulfate can be used together, but typical examples are not disclosed anywhere and any effect is not described. Anyway, the above-mentioned conventional techniques are concerned with the methods for preparing the low-molecular weight polymers, and hence they are different from the present invention in an object.
In the preparation of a high-molecular weight acrylamide polymer, the above-mentioned restrictions are present, and as a result, there are the following problems to be solved. (1) A lengthy period is required for the completion of the polymerization, and therefore some restrictions are put on manufacturing facilities and apparatuses. In addition, productivity is low and the manufacturing cost increases. (2) Since the concentration of the polymerization initiator is low, the repeatability of the polymerization is low, and the time taken until the completion of the polymerization is not constant. (3) The concentration of the polymerization initiator and the concentration of the polymerization monomer are low, and hence the polymerization does not completely come to an end, so that unreacted monomers remain.